QuickBooks Online: Does GET /purchase-orders Support Query By Purchase Order?

No, querying by pOStatus is not a supported feature of the QuickBooks Online API. You can perform a full selection from PurchaseOrder and then loop through the result set to pull out the open order details needed. A custom transformation or formula filter is recommended because the QBO PurchaseOrder API does not support using the postatus='OPEN' within the where clause.